A water pump and the electric motor which operates the pump are submerged in a well of water in a typical installation. In many installations, there is the possibility that the level of the water in the well may drop below the pump in which event the pump and/or the motor will be damaged as a result of improper operation.
Many types of water pump and motor protector devices have been devised to prevent pump and/or pump motor damage. Most of these devices require an element to sense the water level condition, and to provide a current or voltage to indicate a low water condition. Another class of device is strictly mechanical in nature, and uses a float to divert water back into the well. This reduced flow from the well prevents the pump from running dry. All of these devices which utilize some sensing mechanism in the water are prone to reliability problems, primarily as a result of the buildup of minerals from the water on the apparatus.
Another class of devices for pump and pump motor protection senses the changes in the electrical operation of the pump motor when it is pumping water and when it is not pumping water, as the means of detecting a low water condition. Several pump motor operating electrical characteristics can be sensed and used for this purpose. Typically, energy consumed by the pump motor is less when the pump in not pumping water. This change is detected by electronic means, and a switch or relay is actuated to deenergize the circuit to the pump motor when a low water condition is sensed.
U.S. Pat. No. 4,420,787 discloses circuitry which senses changes in power factor to deenergize an electric motor which operates a water pump. Power factor is defined as the cosine of the angle between applied voltage and the current flowing to the motor. When the power factor is used as a method of determining the loading of the pump motor, the phase angle between the voltage and current is measured.
When the phase angle between voltage and current is sensed and used, the customary method is to determine the zero crossings of the voltage and the zero crossings of the current and measure the time between the voltage and current crossings to provide a measure of the phase angle. The phase angle may range between zero (when the voltage and current are in phase), and up to 180.degree. out of phase. The actual phase measurement is accomplished by either measuring the time between zero crossings by counting the number of pulses of a clock between zero crossings, or by developing a series of pulses, the width of which is equal to the distance between the zero crossings, followed by developing a direct current voltage proportional to pulse width, simply by integrating the pulses.
This approach is satisfactory with most alternating current electric motors, particularly those that have a starting winding which is switched out of the circuit after the motor has started (capacitor start, induction run motors.) However, when other motors, such as permanent split capacitor induction motors are used, significant harmonic frequencies may exist in the current waveform. The harmonic frequencies interfere with obtaining an accurate measurement of the zero crossings of the fundamental frequency. Thus, these harmonic frequencies produce errors in the accurate measurement of the phase angle between the voltage and current supplied to the pump motor.
In many pump and motor installations, another problem exists in attempts to accurately deenergize the pump motor when a low water level condition exists. The applied voltage to the pump motor may vary. A variation in applied voltage produces changes in the phase angle, and may result in deenergization of the pump motor even though the pump is delivering water. It is desirable that a change in amplitude of the applied voltage should not produce an indication that a low water level condition exists. Experimental data indicate that the variation is phase angle versus the applied voltage approaches a straight line. Thus, a method of compensating for such voltage variations is necessary to prevent false tripping, or false indications of a low water condition.
It is an object of this invention to provide a pump and electric motor protector mechanism which employs changes in phase angle to deenergize the pump motor when a low water level condition exists.
Another object of this invention is to provide such protector mechanism which includes electrical circuitry which provides a waveform which is related to the current flowing in the electric motor and in which harmonics in this waveform are attenuated. The signal is employed in phase angle measurement. Thus, there are no zero crossing errors which produce errors in the measurement of the phase angle between voltage and current.
It is another object of this invention to provide electrical circuitry which compensates for variations in applied line voltage, so that these variations do not produce false indications of a low water condition.
Other objects and advantages of this invention reside in the combination of elements of the circuitry, the mode of operation, and the methods involved as will become more apparent from the following description.